There is good evidence in the literature that prostaglandins are involved in the initial attachment of the blastocyst to the endometrium and the associated increase in vascular permeability. The present studies will therefore study the ability of developing zygotes and associated endometria to synthesize and metabolize prostaglandins. This knowledge will be important in determining how attachment is regulated, and help to elucidate whether there are differences between species requiring estrogen for induction of implantation (mouse) and those that do not (rabbit).